(a) Field of the Invention
This invention relates to the hydrofromylation of olefins, and to novel catalysts useful in such hydroformylation reactions.
(b) Description of the Prior Art
This process, often referred to as the OXO reaction, involves reaction of olefins with carbon monoxide and hydrogen in the presence of a suitable catalyst. It involves the addition of H and CHO across the double bond of the olefin, hence the term hydroformylation. The aldehyde may be further hydrogenated, either in situ by the same catalyst or in a separate reaction step, as shown in the following reaction scheme: ##STR1## In general, alcohols are the most important products, but aldehydes can be useful chemical intermediates. This is well illustrated in the hydroformylation of propylene, where n-butyraldehyde is both hydrogenated to n-butanol and converted, via aldol condensation, to 2-ethyl-1-hexanol, as shown in the following reaction scheme: ##STR2##
With olefins other than ethylene, a mixture of isomeric aldehydes is produced. From a commercial standpoint, maximum selectivity to the normal, straight-chain product is desirable. The reasons for preferring the n- over the iso- product relate to chemical utility and improved performance in the end product. Thus, in the hydroformylation of propylene, the isobutyraldehyde, readily separable from n-butyraldehyde, is not a useful product and, if present in large amount, may need to be disposed of. For higher molecular weight materials, e.g. C.sub.8 -C.sub.10 alcohols used in plasticiser manufacture, typically as diethylphthalates, and C.sub.12 -C.sub.16 alcohols in detergents, typically as alcohol ethoxylates, RO(CH.sub.2 CH.sub.2 O).sub.x H, separation is difficult and the product mixture is generally used without purification. For detergents, linear alcohols mimic the natural systems with which they compete, e.g. alcohols derived from coconut oil, and are more easily biodegraded than their branched isomers. Price and performance are however always balancing considerations. Thus, for plasticisers, the branched 2-ethyl-hexanol (from C.sub.3 H.sub.6 via n-C.sub.3 H.sub.7 CHO) competes with n-octanol (from the more expensive heptene) largely on the basis of cost as a prime consideration.
The hydroformylation reaction was first operated using supported cobalt catalysts, but it is now known that homogeneous systems (using e.g. homogeneous cobalt or rhodium catalysts) are better than heterogeneous ones.
The patent literature is replete with patents alleging improved catalysts for hydroformylation and improved hydroformylation processes. Among those patents, the following non-exhaustive list may be mentioned.